I've read Watchmen and the movie is the exact story. They change very, very little. It's more omission of some story that would be different.

I wouldn't recommend it to anybody but huge fans of the book. It's slow when it doesn't need to be. I felt the director was more a slave to the visuals of the book instead of using it to guide the action. He overused slow motion and used stationary cameras for long dialogue heavy scenes. It made the nearly 3 hour run time tick by slowly.

It looked EXACTLY like the graphic novel and it was visually precise. Special effects were impressive as well.